package SubjectBacktracking.One;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class LetterCombinations {

/**
 * 难度：中等
 * 
 * 17. 电话号码的字母组合
 * 	给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。
 * 	给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。
 * 	
 * 示例 1：
 * 	输入：digits = "23"
 * 	输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
 * 	
 * 示例 2：
 * 	输入：digits = ""
 * 	输出：[]
 * 	
 * 示例 3：
 * 	输入：digits = "2"
 * 	输出：["a","b","c"]
 * 	
 * 提示：
 * 	0 <= digits.length <= 4
 * 	digits[i] 是范围 ['2', '9'] 的一个数字。
 *
 * */
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		LetterCombinations lc = new LetterCombinations();
		System.out.println(lc.letterCombinations(""));
	}
	//自己写
	Map<Character, String> phoneMap = new HashMap<>() {{
        put('2', "abc");
        put('3', "def");
        put('4', "ghi");
        put('5', "jkl");
        put('6', "mno");
        put('7', "pqrs");
        put('8', "tuv");
        put('9', "wxyz");
    }};
    List<String> result = new ArrayList<>();
    String s = "";
	public List<String> letterCombinations(String digits) {
		if(digits.equals(""))return null;
		backtracking(digits,0);
		return result;
    }
	private void backtracking(String digits,int index) {
		if(index == digits.length()) {
			result.add(s);
			return;
		}
		char digit = digits.charAt(index);		//取出输入的第一个数字
		String letters = phoneMap.get(digit);	//取出输入的第一个数字的字母
		
		for(int i=0;i<letters.length();i++) {
			s+=letters.charAt(i);				// 处理
			backtracking(digits,index+1);		// 递归，注意index+1，一下层要处理下一个数字了
			s = s.substring(0,s.length()-1);	// 回溯
		}
	}
	
	//方法一：回溯
	public List<String> letterCombinations1(String digits) {
        List<String> combinations = new ArrayList<String>();
        if (digits.length() == 0) {
            return combinations;
        }
        Map<Character, String> phoneMap = new HashMap<Character, String>() {{
            put('2', "abc");
            put('3', "def");
            put('4', "ghi");
            put('5', "jkl");
            put('6', "mno");
            put('7', "pqrs");
            put('8', "tuv");
            put('9', "wxyz");
        }};
        backtrack(combinations, phoneMap, digits, 0, new StringBuffer());
        return combinations;
    }
    public void backtrack(List<String> combinations, Map<Character, String> phoneMap, String digits, int index, StringBuffer combination) {
        if (index == digits.length()) {
            combinations.add(combination.toString());
        } else {
            char digit = digits.charAt(index);
            String letters = phoneMap.get(digit);
            int lettersCount = letters.length();
            for (int i = 0; i < lettersCount; i++) {
                combination.append(letters.charAt(i));
                backtrack(combinations, phoneMap, digits, index + 1, combination);
                combination.deleteCharAt(index);
            }
        }
    }
}
